This application relates to a method for reducing the levels of reactive reaction by-products, including in particular linear Fries products and residual monohydric or dihydric phenols, such as bisphenol A (BPA) or phenol in polycarbonate resin compositions.
Polycarbonate resins are commonly formed by a melt transesterification process in which a diaryl carbonate such as diphenyl carbonate is reacted with a dihydric phenol. In the course of this reaction, so-called Fries products may be formed. Fries products may be linear or branched, as shown in FIG. 1, and have been generally recognized as being detrimental to the characteristics of the polycarbonate resin for use in various applications. Thus, for example, Fries products may be associated with reduction in stability, reaction with additives, increased coloration, and loss of transparency, moisture resistance and weather resistance. Further, inclusion of Fries products is associated with a reduction in ductility and impact strength. Branched Fries products can result in polycarbonates which are partially insoluble in conventional solvents such as methylene chloride and lead to an alteration in the melt flow properties. (U.S. Pat. No. 5,468,836)
In addition to Fries products, polycarbonate resins frequently may contain residual dihydric phenol as well as the phenolic reaction product of the transesterification reaction. These phenolic compounds can also detract from the qualities of the polycarbonate resin. For example, polycarbonate compositions with decreased aromatic monohydroxy compounds have been shown to exhibit reduced levels of crazing after exposure to moist heat, less interruption of continuous injection molding processes for cleaning of deposited materials (i.e, less plate-out), and less development of uneven color, for example as a result of deposition of black spots or development of brown streaks. (U.S. Pat. No. 5,852,156). Plate-out has also been associated with high amounts of residual monomers of all types, i.e., phenol, diaryl carbonate and dihydric phenol, in the polycarbonate. (EP 0 709 421)
While reaction by-products such as Fries products and monohydric phenols and residual dihydric phenol have been recognized as undesirable to the quality of the polycarbonate resin, efforts to reduce the amount of such by-products have either been of limited effectiveness or have been cumbersome and expensive. For example, JP 5017565 teaches that levels of impurities like phenol in melt polycarbonate can be reduced by adding the melt polycarbonate to 5 times the weight of dimethylcarbonate, stirring at the boiling point for 5 hours, filtering and then drying the filtered cake for 5 hours at 100 degrees C. EP 0 985 696 discloses a methodology for reduction of linear Fries products to reduce fluorescence. The method makes use of low temperatures, special stirrers and special catalyst to achieve this result. Thus, the reduction in linear Fries products is achieved at the expense of reactor throughput and the molecular weight of the produced resin, but does not address problems associated with residual phenolic monomers. EP 0 709 421 uses acidic components (such as phosphoric acid) to reduce residual monomers, but does not reduce the amount of Fries products.
Thus, there remains a need for a simple and effective method which reduces both the amount of Fries products and the amount of residual monohydric and dihydric phenols in a polycarbonate resin. It is an object of the present invention to provide such a method.
In accordance with the present invention polycarbonate resin containing reduced levels of reactive reaction by-products can be prepared by a method comprising the steps of: (a) reacting a diaryl carbonate and a dihydric phenol in a melt transesterification reaction to produce a composition containing polycarbonate polymer, and one or more reaction by-products selected from the group consisting of linear Fries products and monomeric phenols and one or more residual unreacted dihydric phenol; (b) adding to the composition a scavenging agent to form a mixture, said scavenging agent having the formula: 
wherein R1 is alkoxy, phenoxy, benzyloxy or phenyl, and R2 is a substituted or unsubstituted C1-C30 alkyl group, C6-C30 aryl group, C7-C30 aralkyl group or C6-C30 aryloxy group; (c) processing the mixture at an elevated temperature and for a period of time such that the scavenging agent reacts with linear Fries products to produce capped linear Fries products, and with residual monohydric and dihydric phenols to produce volatile ortho-substituted phenols and capped phenolic species; and(d) separating ortho-substituted phenol, if present, from the processed mixture, thereby producing a polycarbonate resin containing reduced levels of reactive reaction by-products.
Alternatively, the scavenging agent may be added to any polycarbonate resin composition which contains linear Fries products or residual monomeric or dihydric phenols the removal of which would be desirable. Thus, a further aspect of the invention is a method for reducing the amount of such materials in a polycarbonate resin composition comprising the steps of adding to a composition comprising polycarbonate resin and one or more materials selected from the group consisting of linear Fries products and residual monohydric and dihydric phenols a scavenging agent as described above to form a mixture; processing the mixture at an elevated temperature and for a period of time such that the scavenging agent reacts with linear Fries products to produce capped linear Fries products, and with residual phenols to produce volatile ortho-substituted phenols and capped phenolics; and separating the volatile ortho-substituted phenol from the processed mixture.